This invention relates generally to voltage controlled oscillators and more particularly to a voltage controlled oscillator wherein the output circuit is isolated from the tuning circuit.
The use of a varactor having a voltage dependent variable capacitance for tuning resonant circuits is well known in the art. Thus, the level of a bias voltage applied to a varactor in the resonant circuit of an oscillator, such as the well known Clapp oscillator, may be changed to control the frequency of the output of the oscillator. Generally, the output of a Clapp oscillator is taken across a portion of the resonant circuit and, in consequence, output power varies as a function of frequency. If the output power is to be constant as the frequency is changed, a feedback technique such as the technique described in U.S. Pat. Nos. 3,370,254, 3,723,906 or 3,899,755 may be utilized. Although either cited technique is quite effective in providing a tunable oscillator having a relatively constant power output, other considerations of equal importance (such as FM noise generated by the oscillator and the effect of transients on the oscillator) are not addressed by either technique.
If very low FM noise level is to be achieved in the operation of a Clapp oscillator, the Q of the combination of the resonant circuit of the oscillator and a load should be as high as possible. With the load connected directly to the resonant circuit, the Q is limited by the load to such a degree in any practical arrangement that a significant level of FM noise must be tolerated. Further, in the standard Clapp oscillator, any mismatch between the load and the oscillator will result in the phenomenon known as "frequency pulling" or detuning of the oscillator.
The settling time and post tuning drift of an oscillator of the type here being considered is determined primarily by the amount of power dissipated in the varactor used for tuning. In the standard Clapp oscillator the varactor in the resonant circuit is also part of the output circuit. This means that the amount of power dissipated by the varactor will be dependent on the power output of the oscillator. It follows, then, that satisfactory performance of the standard Clapp oscillator may be achieved only at low power levels.